Method for Automatic Piloting of a Rotary Wing Aircraft Having at Least One Thruster Propeller, Associated Automatic Autopilot Device, and Aircraft

ABSTRACT

An autopilot device and method for automatically piloting a rotary wing aircraft, having at least one propulsion propeller, the rotary wing including at least one rotor with blades, the device including a processor co-operating with at least one collective control system for controlling the collective pitch of the blades. The device includes engagement means connected to the processor for engaging an assisted mode of piloting for maintaining an angle of attack, the processor automatically controlling the collective pitch of the blades when the assisted mode of piloting for maintaining an angle of attack is engaged by controlling the collective control system to maintain an aerodynamic angle of attack (α) of the aircraft at a reference angle of attack (α*).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/543,957, filed Jul. 9, 2012, which claims priority to FR 11 02191filed on Jul. 12, 2011; the disclosures of which are incorporated in itsentirety by reference herein.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to a method of and device forautomatically controlling a rotary wing aircraft having at least onepropulsion propeller, to an autopilot device, and to an aircraft.

(2) Description of Related Art

The term “aircraft” includes in particular rotorcraft, i.e. aircraftprovided with a rotary wing, and including helicopters.

For example, a helicopter comprises a rotary wing that includes at leastone main rotor for providing the helicopter with propulsion and lift.

The longitudinal attitude and thus the pitching angle of the helicopterare then necessarily a function of the travel speed of the helicopter.At high speed, a helicopter thus presents a considerable pitching angle,and consequently a strongly nose-down longitudinal attitude.

It can be understood that at high speed, the helicopter presents a largenose-down aerodynamic angle of attack, which angle of attack generatespenalizing aerodynamic drag.

It should be recalled that the longitudinal attitude of a rotorcraftrepresents the pitching angle of the rotorcraft, i.e. the angle betweenthe reference longitudinal axis of the rotorcraft and the orthogonalprojection of the longitudinal reference axis onto a plane normal to thegravity direction.

In contrast, the aerodynamic angle of attack of a rotorcraft representsthe angle between the longitudinal reference axis of the rotorcraft andthe orthogonal projection of the air speed vector of the rotorcraft ontoa vertical plane containing the longitudinal axis.

Finally, the term “air-path slope” designates the angle of the air speedvector of the aircraft relative to its orthogonal projection onto aplane normal to the gravity direction.

Under such circumstances, the longitudinal attitude of a rotorcraft isequal to the algebraic sum of the angle of attack plus the air-pathslope of the aircraft.

Furthermore, certain rotorcraft are also provided with at least onepropulsion propeller.

It then becomes possible to adjust independently the longitudinalforward speed, the vertical speed, and the longitudinal attitude of sucha rotorcraft.

It should be observed that the state of the art includes the documentU.S. 2011/0040431, and the document U.S. 2008/0237392 that describes arotary wing aircraft having a propulsion propeller.

According to document U.S. 2008/0237392, a subsystem of a controlarchitecture provides vertical control and pitching control of theaircraft.

Document WO 99/50611 forms part of a state of the art that is remotefrom the invention, that document WO 99/50611 relating to a control andfiring system.

BRIEF SUMMARY OF THE INVENTION

The present invention thus seeks to propose a method of facilitating theworkload on a pilot of a rotary wing aircraft having at least onepropulsion propeller, the method possibly serving to minimize theaerodynamic drag of the aircraft.

The invention thus provides a method of automatically controlling anaircraft, the aircraft having a rotary wing and at least one propulsionpropeller, the rotary wing comprising at least one rotor provided with aplurality of blades.

This method is remarkable in particular in that during an assisted modeof piloting for maintaining an angle of attack, the aerodynamic angle ofattack of the aircraft is maintained equal to a reference angle ofattack by automatically controlling the collective pitch of the blades.

The workload on the pilot is thus lightened, the pilot possiblyconcentrating solely on controlling two parameters, i.e. thelongitudinal attitude and the longitudinal forward speed, for example,rather than controlling three parameters. In addition, the pilot canmaneuver the aircraft by calling for an angle of attack that isoptimized to minimize the aerodynamic drag of the aircraft.

It should be understood that the term “maintaining the angle of attackof the aircraft equal to a reference angle of attack” is used to meanthat the aerodynamic angle of attack of the aircraft tends to bemaintained equal to a reference angle of attack. As soon as theaerodynamic angle of attack of the aircraft is no longer equal to thereference angle of attack, then action is taken on the collective pitchof the blades to correct that situation.

Conversely, it should be observed that document U.S. 2008/0237392 makesno reference to an angle of attack.

Furthermore, document WO 99/50611 does not suggest an assisted mode formaintaining an angle of attack to maintain the angle of attack of theaircraft equal to a setpoint, but rather it relates to a mode ofmaintaining altitude during which the angle of attack must not exceed alimit so as to make firing possible.

The mode of the invention for maintaining an angle of attack does notseek to maintain the altitude of the aircraft constant, but to maintainthe angle of attack of the aircraft constant.

The method may also include one or more of the followingcharacteristics.

For example, the collective pitch of the blades may be controlledautomatically so as to control a vertical air speed of the aircraft insuch a manner as to maintain the aerodynamic angle of attack of theaircraft equal to a reference angle of attack in application of thefollowing relationship:

α*=θ−arcsin(VZ/TAS)

where “VZ” represents the vertical air speed of the aircraft, “TAS”represents the true air speed of the aircraft, “θ” represents a currentlongitudinal attitude, and “α*” represents the reference angle ofattack.

On such an aircraft, the system of parameters including the longitudinalair speed, the vertical air speed, and the longitudinal attitude may bereplaced by an alternative system including the true air speed of theaircraft, the longitudinal attitude, and the aerodynamic angle of attackof the aircraft.

The air-path slope of the aircraft may then be obtained using thefollowing relationship:

γ=arcsin(VZ/TAS)

where “γ” represents the air-path slope, “VZ” represents the verticalair speed of the aircraft, and “TAS” represents the true air speed.

According to the invention, the following equation is deduced therefrom:

α*=θ−arcsin(VZ/TAS)

where “VZ” represents the vertical air speed of the aircraft, “TAS”represents the true air speed, “θ” represents the current longitudinalattitude, and “α*” represents the aerodynamic angle of attack.

For a given longitudinal attitude, by controlling the collective pitchof the blades, the vertical air speed is modified and thus theaerodynamic angle of attack of the aircraft is modified.

If the forward speed of the aircraft varies and thus if its true airspeed varies, or if the current longitudinal attitude varies, then thecollective pitch of the blades is modified automatically to maintain theaerodynamic angle of attack of the aircraft equal to the reference angleof attack.

According to another aspect, the reference angle of attack may be equalto the current aerodynamic angle of attack of the aircraft at the momentthat the assisted mode of piloting for maintaining an angle of attack isengaged.

This method is then simple, the pilot needing only to adjust a referenceangle of attack prior to engaging the assisted mode of piloting formaintaining an angle of attack.

Furthermore, it is possible to modify the value of the reference angleof attack in flight.

Thus, a pilot may adjust the reference angle of attack if the pilotfinds that necessary.

A new reference angle of attack may be determined using adjustmentmeans, such as:

-   -   a knob arranged on a collective pitch control of a control pitch        control system for controlling the collective pitch of the        blades of the rotary wing, the knob serving to increase or        decrease the collective pitch of the blades when the assisted        mode of piloting for maintaining an angle of attack is not        engaged;    -   control means dedicated to this application; and    -   a knob for engaging a trim actuator arranged on the collective        pitch control system for the blades of the rotary wing, the        current angle of attack at the moment the trim actuator is        engaged becoming the reference angle of attack.

It should be observed that in a first implementation, the pilot acts onthe longitudinal attitude of the aircraft by controlling the cyclicpitch of the blades of the rotary wing. Since the aerodynamic angle ofattack is maintained at a reference angle of attack, by controlling thelongitudinal attitude using a cyclic flight control associated with atleast one cyclic control system for controlling the cyclic pitch of theblades of the rotary wing, the pilot modifies the air-path slope of theaircraft. This provides advantageous piloting comfort.

It should be understood that the term “cyclic flight control” covers aflight control acting on the cyclic pitch of the blades of the rotarywing.

In a second implementation, the longitudinal attitude may be maintainedautomatically equal to a longitudinal reference attitude.

In this second implementation, the aircraft thus has an automatic systemfor maintaining the longitudinal attitude equal to a reference attitude.The reference attitude may be the current attitude of the aircraft atthe moment the attitude maintaining system is engaged, or it may be areference attitude set by means of a knob, for example.

The automatic system may be an autopilot system known as an automaticflight control system (AFCS).

It can be understood that in the following equation:

α*=θ−arcsin(VZ/TAS)

“θ” represents the reference longitudinal attitude.

In one option, the reference longitudinal attitude is automatically keptconstant when a pilot operates a cyclic flight control to modify thelongitudinal attitude.

If the pilot seeks to act on a cyclic flight control associated with atleast one cyclic control system for controlling the cyclic pitch of theblades of the rotary wing to modify the longitudinal attitude, then in abasic variant the attitude is no longer maintained at a referenceattitude by the attitude-maintaining system.

Under such circumstances, when the pilot ceases to act on the cyclicflight control, the automatic system for maintaining attitude isautomatically engaged, with the reference attitude being the referenceattitude as stored prior to the action of the pilot on the cyclic flightcontrol.

Consequently, in this basic version, the reference attitude is notmodified when the pilot operates a longitudinal cyclic flight control tocontrol the longitudinal attitude manually, but the automatic system formaintaining attitude is inhibited.

In a tactical version, the reference longitudinal attitude isautomatically servo-controlled to the current value of the longitudinalattitude whenever a pilot operates a longitudinal cyclic flight controlto modify the longitudinal attitude.

Under such circumstances, when the pilot is no longer acting on thecyclic flight control, the automatic system for maintaining attitude isautomatically engaged, with the reference attitude being the attitudethat is current at the time the attitude-maintaining system re-engages.

Similarly, and optionally, the aerodynamic angle of attack of theaircraft is no longer maintained equal to a reference angle of attackwhen the pilot acts on a collective flight control for controlling thecollective pitch of the blades of the rotary wing.

It should be observed that it is possible, for example, to use forcerods arranged in control linkages to determine whether or not a pilot isoperating a flight control.

Furthermore, the reference angle of attack may be bounded.

For safety reasons, out-of-range or dangerous values are not stored bybounding the reference angle of attack to a range of −4° to +4°, forexample.

In another aspect, a first symbol representing a reference air speedvector may be displayed on an artificial horizon so that a pilot canvisualize a reference air-path slope and the reference angle of attack.

In a first implementation, the longitudinal attitude is controlled bythe pilot of the aircraft, the angle between the current attitudedisplayed on the artificial horizon and the reference air speed vectorthen actually illustrating the reference angle of attack.

In the second implementation, a second symbol representing a referencelongitudinal attitude is displayed on the artificial horizon.

The angle between the first symbol and the second symbol then in factrepresents the reference angle of attack.

Furthermore, it is possible to envisage displaying a third symbol on theartificial horizon representing the current air speed, the current airspeed vector representing the current air-path slope of the aircraft.

Furthermore, it is possible to envisage displaying a fourth symbol onthe artificial horizon representing the current ground speed vector, thecurrent ground speed vector representing the current ground slope of theaircraft.

Optionally, when at least one symbol is displayed on an artificialhorizon to illustrate the reference angle of attack, the color of thesymbol may be modified whenever automatic control of the collectivepitch of the blades to maintain the reference angle of attack requirespower greater than a threshold power, and the reference angle of attackmay be modified automatically to comply with the threshold power.

Since the invention seeks to maintain the aerodynamic angle of attack ata reference angle of attack by acting on the collective pitch of theblades of the rotary wing, there is a risk of going outside the range ofpowers allocated to the rotary wing.

In such a configuration, the reference angle of attack is modified sothat the situation does not occur, and the pilot is informed by changingthe color of at least one symbol illustrating the reference angle ofattack.

For a rotary wing aircraft also having at least one fixed wing, itshould be observed that the symbols displayed for illustrating thereference angle of attack or the current angle of attack enable theangle of attack of the fixed wing of the aircraft to be evaluated andthus make it possible to avoid the fixed wing stalling.

In addition to a method, the invention also provides a deviceimplementing the method.

The invention thus provides an autopilot device for automaticallypiloting a rotary wing aircraft, the aircraft having at least onepropulsion propeller, the rotary wing comprising at least one rotor witha plurality of blades, the device comprising a processor unitco-operating with at least one collective control system for controllingthe collective pitch of the blades.

This device is remarkable in particular in that it includes engagementmeans connected to the processor unit for engaging an assisted mode ofpiloting for maintaining an angle of attack, the processor unitautomatically controlling the collective pitch of the blades when theassisted mode of piloting for maintaining an angle of attack is engagedby controlling the collective control system to maintain an aerodynamicangle of attack of the aircraft at a reference angle of attack.

The device may also include one or more of the following additionalcharacteristics.

The processor unit automatically controlling the collective pitch of theblades for controlling a vertical air speed of the aircraft in such amanner as to maintain the angle of attack of the aircraft equal to areference angle of attack in compliance with the following relationship:

α*=θ−arcsin(VZ/TAS)

where “VZ” represents the vertical air speed of the aircraft, “TAS”represents the true air speed of the aircraft, “θ” represents a currentlongitudinal attitude of the aircraft, and “α*” represents the referenceangle of attack, the device may comprise a set of means connected to theprocessor unit to determine the vertical air speed of the aircraft, thetrue air speed of the aircraft, and the current longitudinal attitude.

For example, the set of means may possess:

-   -   conventional first means for determining the true air speed by        using a Pitot tube or the like;    -   conventional second means for determining the longitudinal        attitude by implementing an attitude heading reference system;        and    -   third conventional means for determining the vertical air speed        by implementing a static pressure takeoff.

In another aspect, the device may include adjustment means connected tothe processor unit for adjusting the reference angle of attack.

Furthermore, the device may include an automatic system for maintaininga longitudinal attitude of the aircraft equal to a reference attitude,the automatic system co-operating with at least one longitudinal cycliccontrol system for controlling the cyclic pitch of the blades of therotary wing.

In a variant, the automatic system may comprise a dedicated computer.

In another variant, the processor unit acts as the automatic system. Theprocessor unit then has authority over the cyclic and collective pitchcontrol systems for the blades.

In addition, the device may include a display connected to the processorunit to display at least one symbol illustrating the reference angle ofattack.

Finally, the invention provides a rotary wing aircraft having at leastone propulsion propeller, the aircraft having an autopilot device asdescribed above.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention and its advantages appear in greater detail from thefollowing description of embodiments given by way of illustration withreference to the accompanying figures, in which:

FIG. 1 is a diagram showing an aircraft;

FIGS. 2 and 3 are diagrams showing a first embodiment; and

FIGS. 4 and 5 are diagrams showing a second embodiment.

Elements present in more than one of the figures are given the samereferences in each of them.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an aircraft 1 having a rotary wing and at least onepropulsion propeller 2.

Each propulsion propeller 2 has a plurality of first blades 2′, inparticular for contributing to the propulsion of the aircraft. It shouldalso be observed that the aircraft may also be provided with a fixedwing 4. The propulsion propellers 2 can be arranged on the fixed wing 4.

Furthermore, the rotary wing 3 comprises at least one rotor 3 having aplurality of second blades 3′, sometimes referred to merely as “blades3′”.

Under such circumstances, the aircraft includes adjustment means 5 foradjusting the pitch of the second blades 3′, the adjustment means 5possibly having at least three servo-controls (not shown) for modifyingthe collective pitch and the cyclic pitch of the second blades 3′.

If the servo-controls extend or retract by the same amount, then thecollective pitch of the blades 3′ is modified. In contrast, if oneservo-control behaves differently from the others, then the cyclic pitchof the second blades 3′ is modified. Reference may be made to theliterature, if necessary, to obtain additional information relating tothe collective pitch and the cyclic pitch of a rotorcraft rotor.

For manual control of the collective pitch of the second blades 3′, theaircraft includes a collective control system 7 connected to theadjustment means. The collective control system 7 then co-operates witha collective flight control 7′, e.g. of the collective pitch lever type.

Furthermore, for manual control of the cyclic pitch of the second blades3′, the aircraft includes at least one cyclic control system 6 connectedto the adjustment means. Each cyclic control system 6 then co-operateswith a cyclic flight control 6′, e.g. of the cyclic stick type.

Thus, a longitudinal cyclic control system makes it possible to requesta modification to the longitudinal trim of the aircraft 1.

Furthermore, the aircraft 1 is also provided with an autopilot device 10that co-operates with the control systems, in particular via parallelactuators or trim actuators that are relatively slow but that can movethrough a large amplitude, and/or via series actuators that arerelatively fast but that can move through only a small amplitude.

The device 10 may also co-operate with an electrical or optical flightcontrol system.

FIG. 2 shows a device 10 in a first embodiment.

Independently of the embodiment, the device 10 comprises a processorunit 15. The processor unit may comprise calculation means 16 thatexecute instructions stored in a memory 17.

As represented by arrow F, the processor unit 15 is then suitable inparticular for controlling a collective control system 7 to modify thecollective pitch of the second blade 3′, e.g. by controlling a parallelactuator and/or a series actuator of the collective control system 7.

To this end, the processor unit 15 may be connected to engagement means20 for engaging an assisted mode of piloting for maintaining an angle ofattack.

During this assisted mode of piloting for maintaining an angle ofattack, the aerodynamic angle of attack a of the aircraft isautomatically maintained equal to a reference angle of attack α* byautomatically controlling the collective pitch of the second blades 3′.It can be understood that the term “the aerodynamic angle of attack α ofthe aircraft is automatically maintained equal to a reference angle ofattack α*” means that the collective pitch of the blades isservo-controlled so that the current aerodynamic angle of attack of theaircraft tends towards the reference angle of attack, or else is equalto the reference angle of attack.

Thus, when a pilot requests implementation of the assisted mode ofpiloting for maintaining an angle of attack by manipulating theengagement means 20, the processor unit 15 automatically controls thecollective pitch of the blades 3 by controlling the collective controlsystem so as to maintain an aerodynamic angle of attack α of theaircraft equal to a reference angle of attack α*.

The reference angle of attack α* may be equal to the current aerodynamicangle of attack α of the aircraft 1 at the moment the assisted mode ofpiloting for maintaining an angle of attack is engaged.

Nevertheless, it is possible to act manually to modify the value of thereference angle of attack α* in flight.

The device 10 may include adjustment means 50 operable by a pilot todefine the value for the reference angle of attack α*.

Independently of the version, it should be observed that the referenceangle of attack may be bounded by the processor unit 15.

In addition, the device 10 may include a display 30 connected to theprocessor unit 15 to display at least one symbol representing thereference angle of attack α*.

To control the collective pitch of the second blades 3′ automatically,the processor unit 15 may control a vertical air speed of the aircraftin such a manner as to maintain the aerodynamic angle of attack of theaircraft equal to a reference angle of attack using the followingrelationship:

α*=θ−arcsin(VZ/TAS)

where “VZ” represents the vertical air speed of the aircraft, “TAS”represents the true air speed of the aircraft, “θ” represents a currentlongitudinal attitude, and “α*” represents the reference angle ofattack.

Under such circumstances, the device 10 has a set of means 25 connectedto the processor unit to determine the vertical air speed of theaircraft, the true air speed of the aircraft, and the currentlongitudinal attitude.

In a first embodiment, the pilot acts on the current longitudinalattitude of the aircraft by cyclically controlling the pitch of thesecond blades 3′ of the rotary wing by means of the cyclic flightcontrol 6′.

Under such circumstances, and with reference to FIG. 3, the display 30may comprise an artificial horizon 35 on which there appears a model 31of the aircraft 1.

The processor unit causes a first symbol 36 to be displayed to enablethe reference angle of attack to be viewed.

The processor unit 15 determines a reference air-path slope γ*corresponding to a reference air speed vector V* and it displays a firstsymbol 36 representing the reference air-path slope γ* and the referenceair speed vector V.

The first symbol 36 is then obtained using the equation:

γ*=θ−α*

Thus, the first space between the model 31 and the horizon H representsthe current longitudinal attitude θ, the second space between thehorizon H and a third symbol 33 represents the reference air-path slopeγ*, and the third space between the model 31 and the third symbol 33represents the reference angle of attack α*.

The pilot can thus easily see this reference angle of attack.

It should also be observed that the processor unit may cause a symbolthat is referred for convenience as the “third” symbol 33 to bedisplayed to represent the current air speed vector of the aircraft. Thefourth space between the horizon H and the third symbol 33 thenrepresents the current air-path slope, the fifth space between the model31 and the third symbol 33 representing the current aerodynamic angle ofattack α.

Furthermore, the processor unit may cause a fourth symbol 34 to bedisplayed that represents the ground speed vector. The sixth spacebetween the horizon H and the fourth symbol 34 then represents thecurrent ground speed.

With reference to FIG. 4, in the second embodiment, the longitudinalattitude θ is automatically maintained equal to a reference longitudinalattitude θ*. The current longitudinal attitude is then equal to thereference attitude.

Consequently, the device 10 then controls an automatic system 40 tomaintain a longitudinal attitude of the aircraft equal to a referenceattitude. The device 10 may include an adjustment member 45 foradjusting the reference attitude, and indeed an engagement member forthis embodiment.

It can be understood that the term “for maintaining a longitudinalattitude of the aircraft equal to a reference attitude” means that thelongitudinal cyclic pitch of the blades 3′ is servo-controlled so thatthe current longitudinal attitude of the aircraft tends towards thereference attitude, or indeed is equal to the reference attitude.

The automatic system 40 co-operates with at least one longitudinalcyclic control system 6 for controlling the cyclic pitch of the bladesof the rotary wing.

Furthermore, the automatic system may be incorporated in the processorunit 15.

The set of means 25 may also include force sensors present in thelongitudinal cyclic control system.

Consequently, in a basic variant, the automatic system 40 does notmodify the reference attitude when the pilot operates a longitudinalcyclic flight control to control the longitudinal attitude manually.Nevertheless, the automatic system for maintaining the attitude is theninhibited.

When the pilot does not act on the cyclic flight control, the automaticsystem for maintaining attitude is engaged automatically, the referenceattitude being the reference attitude stored before the pilot acted onthe cyclic flight control.

In a tactical variant, the reference longitudinal attitude isautomatically servo-controlled on the current value of the longitudinalattitude when a pilot operates a longitudinal cyclic flight control formodifying the longitudinal attitude.

Under such circumstances, when the pilot no longer acts on the cyclicflight control, the automatic system for maintaining attitude is engagedautomatically, with the reference attitude being the attitude that iscurrent at the time the attitude maintaining system is re-engaged.

With reference to FIG. 5, in this second embodiment, the processor unit15 determines a reference air-path slope γ* corresponding to a referenceair speed vector V*, and displays a first symbol 36 representing thereference air-path slope γ* and the reference air speed vector V*.

The first symbol 36 is then obtained from the equation:

γ*=θ*−α*

where θ* represents the reference attitude.

Furthermore, the processor unit may cause a second symbol 32 to bedisplayed to represent the reference attitude θ* together with theabove-described third symbol.

Thus, the first space between the second symbol 32 and the horizon Hrepresents the reference attitude θ*, the second space between thehorizon H and third symbol 33 represents the reference air-path slopeγ*, and third space between the second symbol 32 and the third symbol 33represents the reference angle of attack α*.

The pilot can thus easily visualize this reference angle of attack.

In addition, the processor unit may cause a fourth symbol 34 to bedisplayed to represent the ground speed vector.

Naturally, the present invention may be subjected to numerous variationsas to its implementation. Although several embodiments are describedabove, it will readily be understood that it is not conceivable toidentify exhaustively all possible embodiments. It is possible toenvisage replacing any of the means described by equivalent meanswithout going beyond the ambit of the present invention.

What is claimed is:
 1. A method of automatically piloting a rotary wing aircraft having at least one propulsion propeller provided with a plurality of propulsion propeller blades configured to contribute to propulsion of the aircraft, a rotary wing having a main rotor provided with a plurality of main rotor blades, an adjustment mechanism having a servo-control arrangement configured to adjust pitch of the main rotor blades, and a collective control system connected to the adjustment mechanism and configured to control a collective pitch of the main rotor blades via the adjustment mechanism, the method comprising: during an assisted piloting mode, automatically controlling, by a processor, the collective control system to control the collective pitch of the main rotor blades to maintain an aerodynamic angle of attack (α) of the aircraft equal to a reference angle of attack (α*); and wherein automatically controlling the collective control system to control the collective pitch of the main rotor blades to maintain the aerodynamic angle of attack (α) of the aircraft equal to the reference angle of attack (α*) includes servo-controlling the collective pitch of the main rotor blades via the servo-control arrangement of the adjustment mechanism so that the aerodynamic angle of attack (α) of the aircraft either tends toward the reference angle of attack (α*) or is equal to the reference angle of attack (α*).
 2. The method according to claim 1, wherein the collective pitch of the main rotor blades is controlled automatically to control a vertical air speed of the aircraft in such a manner as to maintain the aerodynamic angle of attack (α) of the aircraft equal to the reference angle of attack (α*) by application of the following relationship: α*=θ−arcsin(VZ/TAS) where “VZ” represents the vertical air speed of the aircraft, “TAS” represents the true air speed of the aircraft, “θ” represents a current longitudinal attitude of the aircraft, and “α*” represents the reference angle of attack.
 3. The method according to claim 1, wherein the reference angle of attack (α*) is equal to a current value of the aerodynamic angle of attack (α) of the aircraft at the moment that the assisted piloting mode is engaged.
 4. The method according to claim 1, further comprising: modifying the reference angle of attack (α*) during flight of the aircraft.
 5. The method according to claim 1, wherein the aircraft further includes a cyclic control system connected to the adjustment mechanism and configured to control a cyclic pitch of the main rotor blades via the adjustment mechanism, the method further comprising: during the assisted mode of piloting, automatically controlling, by the processor, the cyclic control system to control the cyclic pitch of the main rotor blades to maintain a longitudinal attitude (θ) of the aircraft equal to a reference longitudinal attitude (θ*).
 6. The method according to claim 5, wherein automatically controlling the cyclic control system to control the cyclic pitch of the main rotor blades to maintain the longitudinal attitude (θ) of the aircraft equal to the reference longitudinal attitude (θ*) includes servo-controlling automatically the reference attitude (θ*) to a current value of the longitudinal attitude (θ) of the aircraft when a pilot operates the cyclic control system for controlling the cyclic pitch of the main rotor blades so as to modify the longitudinal attitude (θ) of the aircraft.
 7. The method according to claim 5, wherein automatically controlling the cyclic control system to control the cyclic pitch of the main rotor blades to maintain the longitudinal attitude (θ) of the aircraft equal to the reference longitudinal attitude (θ*) includes keeping the reference longitudinal attitude (θ*) constant when a pilot operates the cyclic control system for controlling the cyclic pitch of the main rotor blades so as to modify the longitudinal attitude (θ) of the aircraft.
 8. The method according to claim 1, wherein the reference angle of attack (α*) is bounded.
 9. The method according to claim 1, further comprising: displaying a first symbol representing a reference air speed vector on an artificial horizon so that a pilot can visualize a reference air-path slope and consequently the reference angle of attack (α*).
 10. The method according to claim 9, further comprising: displaying a second symbol representing a reference longitudinal attitude (θ*) on the artificial horizon.
 11. The method according to claim 10, further comprising: displaying a third symbol representing a current air speed vector on the artificial horizon.
 12. The method according to claim 11, further comprising: displaying a fourth symbol representing a current ground speed vector on the artificial horizon.
 13. The method according to claim 9, wherein with at least one symbol being displayed on the artificial horizon to illustrate the reference angle of attack, modifying the color of the symbol whenever automatic control of the collective pitch of the main rotor blades to maintain the reference angle of attack (α*) requires power greater than a threshold power, and modifying automatically the reference angle of attack (α*) to comply with the threshold power.
 14. An autopilot device for automatically piloting a rotary wing aircraft having at least one propulsion propeller provided with a plurality of propulsion propeller blades configured to contribute to propulsion of the aircraft, a rotary wing having a main rotor provided with a plurality of main rotor blades, an adjustment mechanism having a servo-control arrangement configured to adjust pitch of the main rotor blades, and a collective control system connected to the adjustment mechanism and configured to control a collective pitch of the main rotor blades via the adjustment mechanism, the device comprising: a processor configured to, during an assisted mode of piloting, automatically control the collective control system to control a collective pitch of the main rotor blades to maintain an aerodynamic angle of attack (α) of the aircraft equal to a reference angle of attack (α*); and wherein the processor automatically controls the collective control system to control the collective pitch of the main rotor blades to maintain the aerodynamic angle of attack (α) of the aircraft equal to the reference angle of attack (α*) by servo-controlling the collective pitch of the main rotor blades via the servo-control arrangement of the adjustment mechanism so that the aerodynamic angle of attack (α) of the aircraft either tends toward the reference angle of attack (α*) or is equal to the reference angle of attack (α*).
 15. The device according to claim 14, wherein the processor automatically controls the collective control system to control the collective pitch of the main rotor blades to control a vertical air speed of the aircraft in such a manner as to maintain the aerodynamic angle of attack (α) of the aircraft equal to a reference angle of attack (α*) is in compliance with the following relationship: α*=θ−arcsin(VZ/TAS) where “VZ” represents the vertical air speed of the aircraft, “TAS” represents a true air speed of the aircraft, “θ” represents a current longitudinal attitude of the aircraft, and “α*” represents the reference angle of attack, the device further comprises a set of means connected to the processor to determine the vertical air speed of the aircraft, the true air speed of the aircraft, and the current longitudinal attitude.
 16. The device according to claim 14, further comprising adjustment means connected to the processor for adjusting the reference angle of attack (α*).
 17. The device according to claim 14, wherein the aircraft further includes a cyclic control system connected to the adjustment mechanism and configured to control a cyclic pitch of the main rotor blades via the adjustment mechanism, wherein the processor is further configured to, during the assisted mode of piloting, automatically control the cyclic pitch of the main rotor blades to maintain a longitudinal attitude (θ) of the aircraft equal to a reference longitudinal attitude (θ*).
 18. The device according to claim 14, further comprising a display connected to the processor to display at least one symbol illustrating the reference angle of attack (α*).
 19. An aircraft comprising: at least one propulsion propeller provided with a plurality of propulsion propeller blades configured to contribute to propulsion of the aircraft; a rotary wing having a main rotor provided with a plurality of main rotor blades; an adjustment mechanism having a servo-control arrangement configured to adjust pitch of the main rotor blades; a collective control system connected to the adjustment mechanism and configured to control a collective pitch of the main rotor blades via the adjustment mechanism; an autopilot device for automatically piloting the aircraft, the autopilot device including a processor configured to, during an assisted mode of piloting, automatically control the collective control system to control a collective pitch of the main rotor blades to maintain an aerodynamic angle of attack (α) of the aircraft equal to a reference angle of attack (α*); and wherein the processor automatically controls the collective control system to control the collective pitch of the main rotor blades to maintain the aerodynamic angle of attack (α) of the aircraft equal to the reference angle of attack (α*) by servo-controlling the collective pitch of the main rotor blades via the servo-control arrangement of the adjustment mechanism so that the aerodynamic angle of attack (α) of the aircraft either tends toward the reference angle of attack (α*) or is equal to the reference angle of attack (α*).
 20. The aircraft according to claim 19, further comprising: a cyclic control system connected to the adjustment mechanism and configured to control a cyclic pitch of the main rotor blades via the adjustment mechanism; and wherein the processor is further configured to, during the assisted mode of piloting, automatically control the cyclic pitch of the main rotor blades to maintain a longitudinal attitude (θ) of the aircraft equal to a reference longitudinal attitude (θ*). 